1. Field of the Invention
The present invention relates generally to swimming pool heat pumps and more specifically to utilizing a heating element as a component of a swimming pool heat pump.
2. Discussion of the prior art
The prior art swimming pool heat pumps work as follows. The fan draws outside air over the surface of the evaporator, and liquid refrigerant, within the evaporator, expands, thereby forming a gas which draws heat from the outside air. The warm gas then passes through a compressor and the heat increases as the gas is compressed into a liquid as it passes through the condenser, known as the heat exchanger. The cooler swimming pool water is circulated around the condensing coil, absorbing the heat being created in the condenser by the compressing of the gas. The liquid refrigerant then passes through an expansion valve and starts the cycle over again. When there is adequate heat in the ambient air, the process can go on indefinitely, thus heating the pool water.
The problem starts to occur when the ambient air starts to get too cold (usually at a point in the mid 40's). The temperature on the surface of the evaporator drops below freezing, and ice (from the humidity in the air) starts to form on the evaporator. The ice prevents air flow across the evaporator, thus preventing the gas from picking up any heat. The heat pump then shuts down because the heat pump can no longer convert the refrigerant from liquid to gas. The problem is a swimming pool heat pump's inability to continue to supply heat to a swimming pool when the ambient air temperature drops into the mid 40's, the time when a pool requires the most heat.
U.S. Pat. No. 2,847,190 to Slattery discloses a heat pump air conditioner for a house having an auxiliary heater which turns on during a frost condition. The compressor is shut down until the defrost condition is alleviated, at which point is automatically set back into operation.
U.S. Pat. No. 3,366,166 to Gerteis discloses a heating system which heats both air and water using a heat pump.
U.S. Pat. No. 4,543,468 to Shaffer discloses a hot water heater using a heat pump and heating elements. In response to a demand for heat water, indicated by a thermostat, the heating cycle begins. In normal operation, the heating elements are bypassed and the heat pump operates. If an abnormal condition is sensed, such as frost on the evaporator, the heat pump is turned off and the heating element is turned on for the remainder of the cycle.
U.S. Pat. No. 4,550,770 to Nussdorfer et al discloses an air conditioner/heater with low and high wattage heating elements integral to the air conditioning unit. In very cold weather, the compressor is not used, and heat is supplied by the low and high wattage heating elements. In other temperature conditions, the compressor is used and the heating elements are turned off.
U.S. Pat. No. 4,517,807 to Harnish discloses a hot water heater, using both a heat pump and heating elements. In an operating cycle, the heat pump is turned on to heat the water until the water reaches a preset temperature. During the cycle, a thermostat detects whether ice is formed on the heat pump evaporator. In response to frost detection, the heat pump is turned off and the supplemental heat supply consisting of two heating elements within the water tank, is turned on. The heating elements stay on until the water reaches the predetermined temperature. The heat pump remains shut down until the next cycle, in order to extend the operating life of the compressor.
By U.S. Pat. No. 5,205,133 to Lackstrom discloses the need for a supplemental heating system increases when a hot tub is used, since hot tubs are used year-round. The Lackstrom system itself does not use a resistance heater at low temperatures, but uses a power circuit consisting of a heat exchanger and working fluid. A heat pump is used at higher temperatures.